Digitally controlled oscillator for a synthesizer module, synthesizer module, synthesizer, and method for producing an electrical audio signal

ABSTRACT

A digitally controlled oscillator (100), a synthesizer module (200), a synthesizer (300), and a method for producing an electrical audio signal are presented. The oscillator (100) comprises a digital processing unit (10) configured to generate a first pulse wave at a first output (PulseUp) of the processing unit (10), wherein the first pulse wave is arranged to include pulses at at least two different frequencies. The oscillator (100) further comprises a summing circuit (30) and a linear wave shaper (20). The output (PulseUp) of the processing unit (10) is connected to the summing circuit (30) which is arranged to produce a resultant signal based on at least the first pulse wave. The resultant signal is arranged to be fed into the linear wave shaper (20) which is arranged to produce an output signal at the output (OUT) of the oscillator (100) based on modifying the resultant signal.

FIELD OF THE INVENTION

The present invention relates in general to electronic oscillators. Inparticular, however, not exclusively, the present invention concernsdigitally controlled oscillators for use in audio devices.

BACKGROUND

There are known ways to produce saw and pulse waveforms electronicallywith a wide range of frequency control.

A voltage-controlled oscillator (VCO) based on a relaxation oscillatoris essentially a voltage-controlled ramp generator with a reset circuitset to trigger once the ramp reaches a certain threshold. This producesa sawtooth wave which is then wave-shaped to produce other outputwaveforms. A common variation is to switch the direction of integrationto produce a triangle wave instead.

A digitally controlled oscillator (DCO) replaces the reset circuit ofthe VCO with a digitally controlled reset. This brings the advantagethat the frequency can be controlled by pulses subdivided from a veryhigh frequency and very stable clock.

In direct digital synthesis (DDS), the waveform itself is generated as adigital stream of numbers in a digital signal processor (DSP), which arethen fed to a digital-to-analog-converter (DAC) clocked at a sample ratewhich is at least several times the highest desired waveform outputfrequency based on the Nyquist criterion.

If, on the other hand, it is desired to generate several simultaneouswaveforms with independently controllable frequencies, in the case ofthe VCO and the DCO, the analog circuitry must be replicated in itsentirety for each frequency. The DDS method, however, generalizesimmediately to producing multiple waveforms. If sufficient DSP power isavailable, the waveforms can be simply generated separately, and thensummed before the DAC, requiring no further analog components. The DSPrequirements typically scale in proportion to the number of individualwaveforms.

However, in the DDS method, producing the waveform by simply countingthe phase of the oscillator and outputting the correspondinginstantaneous sample value is essentially equivalent to sampling acontinuous time waveform. When the waveform has many harmonics, such assaw and pulse waves, some of these harmonics will be at frequenciesabove the Nyquist bound F_(s)/2, where F_(s) is the sampling frequency,and will alias down to the baseband 0≤f<F_(s)/2. In case of audioapplications, the human hearing is extremely sensitive to thesenon-harmonic frequencies, so the aliasing must be very carefullycontrolled to produce a quality similar to analog sound generationmethods, requiring intensive DSP resources.

SUMMARY

An objective of the present invention is to provide a digitallycontrolled oscillator, a synthesizer module, a synthesizer, and a methodfor producing an electrical audio signal. Another objective of thepresent invention is that the digitally controlled oscillator, thesynthesizer module, the synthesizer, and the method at least alleviatesome of the drawbacks in the known solutions, such as related to thealiasing as described above. Furthermore, the present invention providesa simpler way to produce an electrical audio signal comprising aplurality of audio characteristics.

The objectives of the invention are reached by a digitally controlledoscillator, a synthesizer module, a synthesizer, and a method forproducing an electrical audio signal as defined by the respectiveindependent claims.

According to a first aspect of the present invention, a digitallycontrolled oscillator for a synthesizer module is provided. Thedigitally controlled oscillator comprises a digital processing unit,such as a STM32F103RGT6 microcontroller (MCU), configured to generate afirst pulse wave at a first output, such as at pin PA8 of MCU's TIM1timer. The first pulse wave is arranged to include pulses at at leasttwo different first frequencies, for example, at 75 Hz and 77 Hz. Thedigitally controlled oscillator further comprises a summing circuit anda linear wave shaper, wherein the linear wave shaper comprises anintegrator, such as an essentially ideal integrator or, preferably, anon-ideal, or leaky, integrator. The first output of the processing unitis connected to the summing circuit, and the summing circuit is arrangedto produce a resultant signal based on at least the first pulse wave.The resultant signal is arranged to be fed into the linear wave shaperwhich is arranged to produce an output signal, such as an electricalaudio signal for converting to corresponding sound in a speaker, at theoutput of the oscillator based on modifying the resultant signal,wherein the modifying comprises at least integration of the resultantsignal.

Thus, producing the output signal, preferably, includes, optionallyamong other things, producing the resultant signal which comprises, atleast from a mathematical point of view, a derivative or, optionallyhigher, derivatives, or slopes, of the desired output signal. The linearwave shaper is then arranged to shape, at least by integration, theresultant signal into the desired output signal.

In various embodiments, the digitally controlled oscillator may comprisea DC (direct current)-offset voltage supply, such as a digital-to-analogoutput of the processing unit, connected to the summing circuit forproducing a DC-offset voltage, wherein the resultant signal is furtherproduced based the DC-offset voltage.

In various embodiments, the digitally controlled oscillator may comprisethe digital processing unit configured to generate a second pulse waveat a second output of the processing unit, wherein the second pulse waveis arranged to include pulses at at least two different secondfrequencies. The second output of the processing unit is connected tothe summing circuit, and the summing circuit is arranged to produce theresultant signal at its output based on at least the first and thesecond pulse waves. The resultant signal may, optionally, be based alsoon the DC-offset voltage.

In some embodiments, the at least two different first frequencies maycorrespond to the at least two different second frequencies,respectively.

In various embodiments, the linear wave shaper may comprise an activefilter, preferably an active second order band-pass filter.Alternatively, the linear wave shaper may comprise a passive filterconnected to an amplifier.

In some embodiments, the active filter may comprise a first operationalamplifier, an output of the first operational amplifier is in connectionwith the output of the oscillator, wherein the first operationalamplifier comprises a non-inverting input and an inverting input. Theactive filter may further comprise a first resistor connected betweenthe output of the first operational amplifier and the inverting input.The active filter may further comprise a first capacitor, a firstterminal of which is connected to the inverting input and a secondterminal of the first capacitor is in connection with an input of thelinear wave shaper. Still further, the active filter may comprise asecond capacitor connected between the output of the first operationalamplifier and the second terminal of the first capacitor.

In various embodiments, the summing circuit may comprise a first input,a second input, and, optionally, a third input, wherein the first inputis in connection with the first output of the processing unit and thesecond input is in connection with the DC-offset voltage supply, such asthe digital-to-analog output of the processing unit, and, optionally,the third input is in connection with the second output of theprocessing unit.

In various embodiments, the summing circuit may be arranged to form asingle pulse wave at a common coupling point based on at least the firstpulse wave, a voltage of the DC-offset voltage supply, and, optionally,the second pulse wave, and wherein the resultant signal is based on thesingle pulse wave.

The common coupling point may refer to the point in the electroniccircuit of the oscillator in which at least two of the first pulse wave,the second pulse wave, and the DC-offset voltage, or at least two of thesignals based on the first pulse wave, the second pulse wave, and theDC-offset voltage, are combined into one electrical signal, that is,into the single pulse wave.

In an embodiment, the summing circuit may comprise a first semiconductorswitch, such as a MOSFET. The first semiconductor switch may arranged tobe controlled by its control terminal by the first pulse wave. A firstterminal of the first semiconductor switch may be connected to a secondvoltage supply and a second terminal of the first semiconductor switchmay be connected to the common coupling point.

Alternatively or in addition, the summing circuit may comprise a secondsemiconductor switch, such as a MOSFET. The second semiconductor switchmay be arranged to be controlled by its control terminal by the secondpulse wave. A first terminal of the second semiconductor switch may beconnected to a third voltage supply and a second terminal of the secondsemiconductor switch may be connected to the common coupling point.

In various embodiments, the summing circuit may comprise a digitalbuffer arranged between the processing unit and the common couplingpoint.

In various embodiments, the summing circuit may comprise an invertingamplifier arranged between the common coupling point and the linear waveshaper.

According to a second aspect of the present invention, a synthesizermodule is provided. The synthesizer module comprises at least onedigitally controlled oscillator according to the first aspect or anyembodiment thereof. The synthesizer module further comprises a userinterface for controlling the at least one digitally controlledoscillator, wherein the user interface is at least in functionalconnection with an input of the at least one digitally controlledoscillator. For example, the user interface may comprise a plurality ofrotatable knobs for controlling, in response to rotating of a knob, thesetting(s) of the oscillator, such as related to the waveform of theoutput signal of the oscillator. In various embodiments, there may bearranged MIDI (Musical Instrument Digital Interface) as the userinterface for controlling the operation of the oscillator.

According to a third aspect of the present invention, a synthesizer isprovided. The synthesizer comprises at least one digitally controlledoscillator according to the first aspect or any embodiment thereof. Thesynthesizer further comprises a keyboard, such as a musical keyboard,including a plurality of keys, wherein the keyboard is at least infunctional connection with an input of the at least at least onedigitally controlled oscillator, and, optionally, a speaker inconnection with the output of at least at least one digitally controlledoscillator.

According to a fourth aspect of the present invention a method forproducing an electrical audio signal is provided. The method comprisesat least the steps of:

-   -   generating a first pulse wave by a digital processing unit, the        first pulse wave comprising pulses at at least two different        first frequencies,    -   producing a resultant signal based on at least the first pulse        wave, and    -   filtering the resultant signal by a linear wave shaper, such as        comprising an active filter or a passive filter in connection        with an amplifier, for generating the electrical audio signal.

In some embodiments, the method may comprise generating a second pulsewave by the digital processing unit, the second pulse wave comprisingpulses at at least two different second frequencies, and producing theresultant signal based on at least the first pulse wave and the secondpulse wave.

The present invention provides advantages over known solutions in that aplurality or, preferably, a large number of independent waveforms can begenerated with no or at most with a small increase in the componentcount. Furthermore, the high frequency resolution and absence or lowlevel of aliasing typical of the DCO based solutions are retained.

Various other advantages will become clear to a skilled person based onthe following detailed description.

The expression “a number of” may herein refer to any positive integerstarting from one (1).

The expression “a plurality of” may refer to any positive integerstarting from two (2), respectively.

The terms “first”, “second”, “third”, “fourth” etc. are herein used todistinguish one element from other element, and not to speciallyprioritize or order them, if not otherwise explicitly stated.

The exemplary embodiments of the present invention presented herein arenot to be interpreted to pose limitations to the applicability of theappended claims. The verb “to comprise” is used herein as an openlimitation that does not exclude the existence of also un-recitedfeatures. The features recited in depending claims are mutually freelycombinable unless otherwise explicitly stated.

The novel features which are considered as characteristic of the presentinvention are set forth in particular in the appended claims. Thepresent invention itself, however, both as to its construction and itsmethod of operation, together with additional objectives and advantagesthereof, will be best understood from the following description ofspecific embodiments when read in connection with the accompanyingdrawings.

BRIEF DESCRIPTION OF FIGURES

Some embodiments of the invention are illustrated by way of example, andnot by way of limitation, in the figures of the accompanying drawings.

FIG. 1 illustrates highly schematically a digitally controlledoscillator according to an embodiment of the present invention.

FIG. 2 illustrates schematically a digitally controlled oscillatoraccording to an embodiment of the present invention.

FIG. 3 illustrates schematically a digitally controlled oscillatoraccording to an embodiment of the present invention.

FIG. 4A illustrates Dirac deltas with integral given by the length ofthe arrow.

FIG. 4B illustrates schematically approximations of the Dirac deltas byfinite width pulses in accordance with an embodiment of the presentinvention.

FIGS. 5A and 5B illustrate schematically combining of overlapping pulsesin accordance with some embodiments of the present invention.

FIG. 6 illustrates schematically a synthesizer module according to anembodiment of the present invention.

FIG. 7 illustrates schematically a synthesizer according to anembodiment of the present invention.

FIG. 8 illustrates a flow diagram of a method according to an embodimentof the present invention.

DETAILED DESCRIPTION OF SOME EMBODIMENTS

FIG. 1 illustrates highly schematically a digitally controlledoscillator 100 according to an embodiment of the present invention. Thedigitally controlled oscillator 100 comprises a processing unit 10, suchas an MCU, and a linear wave shaper 20, such as including an activefilter. The processing unit 10 may be connected to the linear waveshaper 20 via a summing circuit 30. The summing circuit 30 is arrangedto sum or combine a plurality of signals at the input 15 of the summingcircuit 30 to form a resultant signal at the output 25 of the summingcircuit 30. The resultant signal at 25 is furthermore arranged to be fedinto the linear wave shaper 20 for obtaining an output signal at theoutput OUT of the oscillator 100. Furthermore, there may be oscillatorinputs 101, such as to be arranged in connection with a user interfaceof a synthesizer module and/or with a keyboard of a synthesizer.

FIG. 2 illustrates schematically a digitally controlled oscillator 100according to an embodiment of the present invention. In FIG. 2, GNDrefers to the ground potential and OUT to the output of the oscillator100.

In FIG. 2, the oscillator 100 may comprise a processing unit 10, such asa microcontroller (MCU). The MCU may utilize an external 8 MHz crystalto produce a main clock rate of 72 MHz. In an embodiment, the MCU maybe, for example, STM32F103RGT6 microcontroller manufactured bySTMicroelectronics® or the like.

For the first operational amplifier U1A, a quarter of TL074 operationalamplifier or the like may be utilized. The operational amplifier may bechosen based on the desired properties thereof.

The first terminals of the resistors R1, R2, and R3 may be connected tothe inputs DAC, PulseUp, and PulseDown, respectively, of the summingcircuit 30. The second terminals of the resistors R1-R3 may be connectedto each other as can be seen in FIG. 2.

The linear wave shaper 20 may be implemented by the first operationalamplifier U1A and the surrounding passive network therein comprisingresistor R4 and capacitors C1 and C2.

In some embodiments, the capacitor C1 may be part of the summing circuit30 or be common to both the summing circuit 30 and the linear waveshaper 20.

In an exemplifying embodiment, such as shown in FIG. 2, the componentvalues may be as follows: R1=R2=100 kΩ, R3=278 kΩ, R4=820 kΩ, C1=330 nFand C2=40 nF. The operation of the oscillator is dependent on the ratiosof the component values, so multiplying all component values by the samepositive real number provides an oscillator 100 that works essentiallythe same way as the one with the abovementioned component values.

Furthermore, as can be seen in FIG. 2, the processing unit 10 may beconfigured to generate at least two output signals, namely, PulseUpand/or PulseDown, and DAC.

In an embodiment, the PulseDown and PulseUp signals may be generated bythe MCU's TIM1 and TIM8 timers PWM outputs on pins PA8 and PC6,respectively. The DAC output may be generated from the MCU's internalDAC2, on output pin PA5. The DAC output may be further buffered by avoltage follower using, for example, a quarter of a TL074 operationalamplifier (not shown).

FIG. 3 illustrates schematically a digitally controlled oscillator 100according to another embodiment of the present invention. The oscillator100 may, preferably, be used as an oscillator in a “Eurorack”-formatmodular audio synthesizer. In FIG. 3, the oscillator 100 comprises anMCU, such as the STM32F103RGT6 microcontroller, utilizing an external 8MHz crystal to produce a main clock rate of 72 MHz.

Furthermore, the PulseDown and PulseUp signals may be produced by theMCU's TIM1 and TIM8 timers PWM outputs on pins PA8 and PC6,respectively. TIM1 and TIM8 timers PWM outputs and pins PA8 and PC6, perse, are known to a person skilled in art. The DAC output may begenerated from the MCU's internal DAC2 on output pin PA5, and bufferedby a voltage follower using a quarter of a TL074 operational amplifier(not shown).

In some embodiments, the linear wave shaper 20 may be implemented withany of the standard op-amp bandpass filter configurations, such asSallen-Key, state variable, ladder with feedback and so on.

In preferable embodiments, the linear wave shaper 20 may be based on amultiple feedback (MFB)-topology. In FIG. 3, the linear wave shaper 20is implemented by operational amplifier U1A and the surrounding passivenetwork comprising resistors R10, R11 and capacitors C3 and C4.

In an embodiment, such as shown in FIG. 3, the component values may beas follows: R5=82 kΩ, R6=56 kΩ, R7=22 kΩ, R8=22 kΩ, R9=47 kΩ, R10=18 kΩ,R11=820 kΩ, and C3=330 nF, C4=8.2 nF, and, an optional component, C5=33pF.

In case of FIG. 3, the cutoff, asymptotic gain at large w and Q of thefilter of the linear wave shaper 20 may be approximated by equations(EQ1)-(EQ3) as follows:

$\begin{matrix}{{\omega_{C} = {\frac{1}{\sqrt{R10R11C3C4}} \approx {2{\pi 25}\mspace{14mu}{Hz}}}},{and}} & ({EQ1}) \\{{{Q = {\frac{\sqrt{\frac{R11}{R10}C3C4}}{{C3} + {C4}} \approx 1}},{and}}\ } & ({EQ2}) \\{\omega_{g} = {{- \frac{1}{R10C4}} \approx {{- 2}{\pi 1080}\mspace{14mu}{{Hz}.}}}} & ({EQ3})\end{matrix}$

Furthermore, the linear wave shaper 20 may be fed by the summing circuit30 around the operational amplifier U1B. In the summing circuit 30, theDAC signal may arranged to be amplified, such as at least partly basedon the ratio of R9 to R5. The PulseDown and PulseUp signals from theprocessing unit 10, such as an MCU, are configured to control MOSFETswitches Q1A, Q1B, for example, a DMN63D8LDW logic-level gate MOSFETpair, for connecting the analog supply voltage(s) U2 and U3, for example3.3 V, at the summing circuit 30. By utilizing the switches Q1A and Q1B,connecting the noisy digital supply (which provides power to the MCU)directly to the audio circuit, which can produce detectableinterference, is being avoided. In FIG. 3, U1 is a first voltage supplysupplying, for example, −10 V.

In the example of FIG. 3, when PulseUp is high (that is Q1A isconducting), it shifts the output voltage of U1B by −3.3 V multiplied bythe ratio of R9 to R7, and the same holds for PulseDown, however, withthe ratio of R9 to R8. Finally, R6 uses U1 of −10 V voltage reference toshift the output down, such that when all of the pulses are in theirinactive state (low for PulseDown, high for PulseUp) and the DAC is atmid-value, the summing circuit 30 output is approximately at groundpotential GND, or somewhat higher, at about 0.4 V, to allow for maximumheadroom. The absolute voltage values presented herein depend, ofcourse, on the particular choice of the component values.

Additionally, the optional component C5 may be arranged to limit theamplifier bandwidth, for example, to about 100 kHz, to suppress veryhigh frequency interference from the digital side of the oscillator 100.

In the configuration of FIG. 3, when PulseUp pulses to its active-lowstate, it produces an output pulse of about 7 V (inverting amplifier),and when PulseDown pulses to its active-high state, it produces a −7 Vpulse. The DAC has about plus/minus 1 V control range. The small DC(direct current) voltage offset due to the inexact shift playsinsignificant role, since the linear wave shaper 20 has zero DCresponse. For U1B, a quarter of a TL074 operational amplifier may beused, with the remaining half buffering the MCU DAC output and providingthe final line driver and amplifier for the output stage of theoscillator 100.

The oscillator 100 according to various embodiments may configured togenerate sawtooth, pulse and/or various other waveforms at its outputOUT.

According to an embodiment, the oscillator 100 may be configured togenerate a sawtooth waveform at its output OUT. This may be implementedby arranging an impulse train superimposed by a small DC-offset. In ananalog system, a train of finite pulses (generated by a clock circuit oran MCU) may be used to approximate the Dirac deltas. This principle isillustrated in FIGS. 4A and 4B which schematically illustrate derivativeof the ideal sawtooth waveform. The arrows in FIG. 4A depict Dirac deltafunctions with integral given by the length of the arrow. FIG. 4B showsapproximation of Dirac deltas by finite width pulses. The finite pulsesmay be in the range of a duty cycle from about 0.01 to 0.2, forinstance. In case of FIG. 4B, the duty cycle is about 0.05.

There is an approximation error which stems from the finite width of thepulses, and the finite time resolution of the pulse times. In audioapplication, the highest frequency of interest may be taken to bef_max=20 kHz. In this case the maximum approximation error for a 0.5-1μs pulse, that is with a duty cycle of 0.01-0.02, is approximately lessthan or equal to −0.006 dB. Even for a 10 μs pulse, that is with a dutycycle of 0.2, the error is approximately equal to −0.6 dB.

In order to keep a constant approximation error, the oscillator 100 maybe configured to keep the pulse length (as opposed to the duty cycle)fixed even when the oscillator frequency varies. The finite timeresolution error may be removed by quantizing the period to the closestinteger number of clock cycles. In the case of audiorate applications,the highest periods of interest are some kilohertz, and the audio rangeextends up to 20 kHz. Then an oscillator 100 having the main clock rateof 72 MHz allows a frequency resolution of one part in 36000 at 2 kHz orabout 0.05 cents in musical notation, which is considerably better thanthe resolution of human pitch perception. Therefore, the embodiments ofthe present invention work very well for audiorate systems, and couldeasily be extended at least some octaves above audio rates in otherapplications.

In order to generate the DC offset part, which is frequency dependent, avoltage proportional to the frequency is generated from a DAC. At aconstant oscillator frequency, the DAC output is constant, so theoscillator 100 remains alias free regardless of the sample rate of theDAC. If the frequency is varied, then the frequency control signal isaliased as determined by the DAC's sample rate, but there is stillminimal or no further aliasing in the oscillator 100 according tovarious embodiments of the present invention.

After generation of the pulses as illustrated, for instance, in FIG. 4B,the pulse train may, preferably, be converted into a sawtooth wave.

The conversion may be implemented in an embodiment by a linear waveshaper 20 comprising an integrator in which there may be an integratorpole which is slightly offset from the origin (in other words, theintegrator is leaky) in the Laplace plane with respect to an idealintegrator, changing the transfer function from ω_(g)/s to ω_(g)/(s+p₀),where p₀ is a positive real number which is small compared to thesmallest frequency of interest.

According to a preferable embodiment, the linear wave shaper 20 maycomprise an integrator that may be implemented by placing a zero at theorigin and replacing the pole by a complex pair of poles, thus giving asecond-order bandpass response, which may be described by equation (EQ4)as follows:

$\begin{matrix}{{{H(s)} = \frac{k\frac{s}{\omega_{c}}}{1 + {\frac{1}{Q}\frac{s}{\omega_{c}}} + \frac{s^{2}}{\omega_{c}^{2}}}},} & ({EQ4})\end{matrix}$

where ω_(c) is the cutoff (angular) frequency, Q determines the amountof peaking in the response and the gain k=ω_(g)/ω_(c), in order to haveasymptotic gain of the same order at high frequencies. Furthermore, theratio of the response H(s) to the ideal response ω_(g)/s is

$\begin{matrix}{{H(s)} = {\frac{k\frac{s^{2}}{\omega_{c}^{2}}}{1 + {\frac{1}{Q}\frac{s}{\omega_{c}}} + \frac{s^{2}}{\omega_{c}^{2}}}.}} & ({EQ5})\end{matrix}$

Therefore, adding the low frequency zero effectively filters the outputsignal with a second order high-pass filter (HPF) relative to the idealcase. The parameter ω_(c) is the cutoff point of the HPF, and Qdetermines the peaking of the response.

Choosing ω_(c) may be based on choosing the lowest frequency ofinterest, f_min. The choice may be affected by the fact that the higherthe cutoff, the faster the DC offset errors settle (and the smaller thetotal energy in them).

Choosing Q may be based on reducing the total amplitude or energy withresponse to a step input. As an example, numerically minimizing the peakvalue of the step response of the filter, with a fixed ω_(c), givesQ=1.354.

In various embodiments, the oscillator 100 comprising the linear waveshaper 20 as described above may produce an output signal which may beused to generate a sound that includes a slight bass boost and a slighthigh frequency droop which may be considered beneficial, depending onthe application. The sound is, thus, slightly more warm and solid thanthat with an oscillator having a mathematically ideal integrator.

According to various embodiments of the present invention, the essentialpoint is that the wave shaper 20 is substantially linear. This allowsmoving the summing unit 30 to the front of the wave shaper 20, i.e. thepulses generated by the processing unit may be combined, such as summed,in the summing circuit 30 and then advantageously only one wave shaper20 may be utilized for modifying the resultant signal in the output ofthe summing circuit 30 and which is being fed to the wave shaper 20.

Therefore, in contrast to known solutions in which N pulsetrains and NDAC signals are fed to N identical wave shapers, whose outputs are thensummed to produce the final output waveform, the N DAC signals can invarious embodiments of the present invention be combined by summing theoffset voltages digitally before the conversion in the processing unit10.

Thus, the hardware needed for generating N sawtooths, for instance, canbe reduced to a single linear wave shaper 20, single DAC for producingDAC signal, and N pulse wave generators. This is may be implemented, forexample, by a STM32-series microcontrollers that have up to 12independent timers with PWM generation and a built-in 12-bit DAC, sosuch a controller equipped with the wave shaper 20 and a summing circuit30 comprising a simple operational amplifier for the DAC and PWM outputscould generate 12 sawtooths with independent frequencies and amplitudes.

In known solutions, concerning the summing of the pulses, assuming thepulse width multiplied by the number of oscillators is small relative tothe oscillator periods, most of the time simply 0 is being added to 1,since at any given time usually only one of the pulses is in its activestate. Therefore, in various embodiments of the present invention, thepulses may be combined by interleaving the pulse trains in theprocessing unit 10 digitally to produce a single pulse train foroutputting from a single pin of the processing unit 10.

However, in the rare case that the pulses do overlap, in variousembodiments, one or more of the overlapping pulses are not ignored,since that would produce a DC error of the same amplitude as thewaveform to be generated. While the error would decay away due to thewave shaper 20, a glitch due to the omission of one or more of theoverlapping pulses would be clearly detectable.

In various embodiments, the lengths of the overlapping pulses may beadded so that a single longer pulse is produced.

The addition to form the single longer pulse may in some embodiments beimplemented by starting the longer pulse at the same time as the firstof the overlapping pulses 51 to be combined, and then continue for anextended time to settle at the correct DC-level. In this case, the errorpulse 52 is entirely positive, contributing mostly low-frequency energy.This is illustrated in FIG. 5A in which three perfectly overlappingpulses 51 are being combined into single longer pulse 53, each of theoverlapping pulses being one microsecond long. FIG. 5A shows that thelonger pulse 53 that starts at the pulse time of the overlapping pulses51. The amplitude in FIG. 5A is relative to the sawtooth peak-to-peakamplitude.

According to another embodiment, the addition to form the single longerpulse 56 may be implemented by shifting the combined pulse 56 to occursymmetrically around the original, overlapping pulse 54 times, thuseliminating the low-frequency part of the error 55. This is illustratedin FIG. 5B which shows the longer pulse 56 occurring symmetricallyaround the overlapping pulses 54. In FIG. 5B, three perfectlyoverlapping pulses 54 are being combined into one longer pulse 56, eachof the overlapping pulses 54 being one microsecond long. The amplitudein FIG. 5B is relative to the sawtooth peak-to-peak amplitude.

The embodiment, the operation of which is illustrated in FIG. 5B,provides the advantage that in the non-symmetric case of FIG. 5A, theerror due to the addition of pulse lengths of the overlapping pulsesproduces a sound much like a delta function, i.e. a sharp full-bandwidthclick. In the symmetric case of FIG. 5B, the sound due to the errorwould be a much subdued, very high frequency blip if any of its energyeven manifests in the audible region.

In the audio applications, no audible degradation occurs as long as theoverlaps of the pulses are relatively infrequent. If they become morefrequent, such as due to increasing frequency or increasing the numberof oscillators in the system, in the embodiment of FIG. 5A, the errorsmay manifest as a vinyl-like crackle. This may be further alleviated byimplementing the addition of the overlapping pulses as in FIG. 5B.

The oscillator 100 according to various embodiments may, alternative orin addition, be configured to generate pulse waves.

The pulse wave may be produced by adding another pin that feeds the waveshaper to produce a falling edge of the pulse wave. In an embodiment,this may be implemented by an inverting amplifier. In effect, twostreams of pulses, each output from their own pin of the processing unit10, such as microcontroller may be generated.

According to some embodiments, in which the linear wave shaper 20rejects any DC-offsets, the inverting amplifier may be omitted, if any,and feed the other pin also directly, simply making the pulseactive-low. In other words, when not outputting a pulse for generating afalling edge, the pin of the processing unit 10 may be at the operatingvoltage of the processing unit 10. Thus, as the DC-offset gets removedby the wave shaper 20, after which a brief pulse where the pin istemporarily connected to ground manifests as a downward pulse.

The oscillator 100 according to various embodiments may be configured togenerate also other waveforms than sawtooth and pulse waves.

The oscillator 100 may be configured to produce any combination of asingle ramp superposed with rising and falling edges, whose heights canbe controlled by adjusting the pulse duration. Further, the directionsand magnitudes of the ramp part may be configured to be changed,producing waveforms such as the triangle, and more. Indeed, in someapplications this enables to specify complicated waveforms that arepiecewise linear with vertical edges.

According to an embodiment, the processing unit 10 may be a STM32F103which features a DAC suitable for use to implement complicated waveformsas described above.

Various embodiments of the present invention allow a much lower samplerate for the DAC (only proportional to the highest desired oscillatorperiod, not to the highest sinusoidal component of interest), andrequires no explicit anti-aliasing in the software.

The software in the processing unit 10 may be given a list ofoscillators (for example, up to 16), each having a waveform, period innumber of ticks of the master 72 MHz clock, phase, and an amplitude,given in terms of the pulse width.

As an example, a sawtooth waveform only may be considered, however, thepulse waveform simply requires the other pulse generator to be drivenwith opposite polarity to produce the falling edges. The processing maybe arranged to occur in 0.5 ms blocks. For each frequency, the next edgetime can be determined from the period and the current phase. Thefrequency related data may be kept in a binary heap, sorted according tothe next edge time. The top of the heap is popped, the correspondingedge (consisting of start time and duration) is added to an edge list,the oscillator phase is advanced by one cycle and it is reinserted tothe heap. If the current edge overlaps with the previous one, the edgesmay be combined by adding their lengths. This is repeated until the nextedge time is later than the boundary of the current 0.5 ms block.Oscillator parameters may be updated between the processing blocks.

The edge list allows building a list of wait-times between pulses andlengths of individual pulses. These may be fed to TIM8 compare channel1, using DMA and the burst mode provided in the timer.

Equations concerning the operation of the oscillator 100 according tosome embodiments, such as shown in FIG. 3, may be approximated asfollows. If a pulse of length t_pulse is fed into the linear wave shaper20, the wave shaper 20 outputs an edge of heightV_p−p=t_pulse*ω_(g)*V_pulse, where V_pulse=−3.3 V*(R9/R7), ω_(g) isgiven in (EQ3). This is an edge of the sawtooth wave. It also gives thepeak-to-peak voltage of the output waveform for a single oscillator 100.In case of a 3 μs pulse as maximum, this gives a maximumpeak-to-peak-voltage of about 140 mV.

Requiring that there is no accumulated DC shift during the period of theoscillator 100 gives an equation for the DAC output value according towhich V_p−p=t_period*ω_(g)*V_DAC, where t_period=1/f, the period of thesawtooth being generated and V_DAC is the amount by which the DAC mustshift the output of the pulse summing circuit 30. This givesV_DAC=(t_pulse/t_period)*V_pulse.

When multiple oscillators are running the DAC output code is simply thesum of the corresponding codes. Note that the choice of the ratio(R5/R7), the pulse width (i.e. amplitude) together with the maximumnumber of simultaneous oscillators determines the frequency range atwhich the DAC is able to compensate the shifts. With our values, therange is 3 kHz for 16 oscillators and pulse width of 3 μs.

In various embodiments, in which the linear wave shaper 20 has zero DCresponse, an offset may be imposed on the DAC and effectively use it asbipolar, allowing for rising and falling ramps.

FIG. 6 illustrates schematically a synthesizer module 200 according toan embodiment of the present invention. The synthesizer module 200 maycomprise at least one or a plurality of digitally controlled oscillators100 according to an embodiment of the present invention. The synthesizermodule 200 may further comprise a user interface 150 for controlling theat least one or the plurality of digitally controlled oscillator 100,wherein the user interface 150 is at least in functional connection withan input 101 of the at least one or inputs of the plurality of digitallycontrolled oscillators 100.

The user interface 150 of the synthesizer module 200 may, according tovarious embodiments may, comprise several means for controlling, such asknobs, the operation of the oscillator(s) 100. These may be arranged toselect a waveform of the output signal of the oscillator 100, such as asawtooth or a pulse wave, for instance. The user interface 150 mayfurther include various inputs or outputs such as in known synthesizermodules 200.

In various embodiments, the synthesizer module 200 may comprise ahousing into which its components have been arranged into. The userinterface 150 may be arranged into the outer surface of the housing.

FIG. 7 illustrates schematically a synthesizer 300 according to anembodiment of the present invention. The synthesizer 300 may comprise atleast one or a plurality of digitally controlled oscillators 100according to an embodiment of the present invention. Alternatively, thesynthesizer 300 may comprise a keyboard 160, such as a musical keyboard,including a plurality of keys, wherein the keyboard 160 is at least infunctional connection with an input 101 of the at least at least onedigitally controlled oscillator 100. Optionally, there may be a speaker250 in connection with the output OUT of at least at least one or aplurality of digitally controlled oscillators 100.

Alternatively, the synthesizer 300 may comprise a synthesizer module 200according to some embodiment of the present invention. Thus, thesynthesizer 300 may comprise the user interface 150 of the module 200.The user interface 150 may, preferably, be separate with respect to thekeyboard 160.

Furthermore, the keyboard 160 may be arranged in an integrated manner orseparately, notwithstanding the electrical connections to the module200.

FIG. 8 illustrates a flow diagram of a method according to an embodimentof the present invention.

It is to be noted, however, that what is described hereinabove withrespect to FIGS. 1-7 may also apply to methods according to someembodiments of the present invention. For example, the components usedand the way they are utilized as described therein may form part of anembodiment of the method. The methods according to various embodimentsof the present invention should not, however, be interpreted to belimited only to what is presented with respect to FIGS. 1-7.

Thus, item 1001 may refer to a start-up phase of the method. Suitableequipment and components, such as, but not limited to, oscillator(s)100, is/are obtained and systems, for example, an arrangement includingthe oscillator(s) 100, a synthesizer module 200, a synthesizer 300,assembled and configured for operation.

Item 1010 may refer to generating a first pulse wave by a digitalprocessing unit 10, the first pulse wave comprising pulses at at leasttwo different first frequencies.

According to an embodiment, the method may comprise generating a secondpulse wave by the digital processing unit 10, the second pulse wavecomprising pulses at at least two different second frequencies.

Item 1020 may refer to producing a resultant signal based on at leastthe first pulse wave.

In some embodiments, the resultant signal may, alternatively or inaddition, be based on at least the first pulse wave and the second pulsewave.

Item 1030 may refer to filtering the resultant signal by an activefilter for generating the electrical audio signal.

Method execution may be stopped at item 1099. The electrical audiosignal may be fed or injected into a speaker or loudspeaker forconverting the electrical audio signal into sound.

The specific examples provided in the description given above should notbe construed as limiting the applicability and/or the interpretation ofthe appended claims. Lists and groups of examples provided in thedescription given above are not exhaustive unless otherwise explicitlystated.

1. A digitally controlled oscillator for a synthesizer module, whereinthe oscillator comprises: a digital processing unit configured togenerate a first pulse wave at a first output of the processing unit,wherein the first pulse wave is arranged to include pulses at at leasttwo different first frequencies; a summing circuit; wherein the firstoutput of the processing unit is connected to the summing circuit, andthe summing circuit is arranged to produce a resultant signal based onat least the first pulse wave; and wherein the resultant signal isarranged to be fed into the linear wave shaper, wherein the linear waveshaper is arranged to produce an output signal, being an electricalaudio signal including, as fundamental frequency components thereof,frequency components corresponding to the at least two different firstfrequencies, at the output of the oscillator based on modifying theresultant signal, wherein the modifying comprises at least integrationof the resultant signal.
 2. The digitally controlled oscillator claim 1,comprising a DC-offset voltage supply, such as a digital-to-analogoutput of the processing unit, connected to the summing circuit forproducing a DC-offset voltage, wherein the resultant signal is furtherproduced based the DC-offset voltage.
 3. The digitally controlledoscillator claim 1, further comprising the digital processing unitconfigured to generate a second pulse wave at a second output of theprocessing unit, wherein the second pulse wave is arranged to includepulses at at least two different second frequencies, wherein the secondoutput of the processing unit is connected to the summing circuit, andthe summing circuit is arranged to produce the resultant signal at itsoutput based on at least the first and the second pulse waves, andoptionally the DC-offset voltage.
 4. The digitally controlled oscillatorof claim 3, wherein the at least two different first frequenciescorrespond to the at least two different second frequencies,respectively.
 5. The digitally controlled oscillator of claim 1, whereinthe linear wave shaper comprises an active filter, preferably an activesecond order band-pass filter.
 6. The digitally controlled oscillator ofclaim 5, wherein the active filter comprises: a first operationalamplifier, an output of the first operational amplifier is in connectionwith the output of the oscillator, wherein the first operationalamplifier comprises a non-inverting input and an inverting input; afirst resistor connected between the output of the first operationalamplifier and the inverting input; a first capacitor, a first terminalof which is connected to the inverting input, wherein a second terminalof the first capacitor is in connection with an input of the linear waveshaper; and a second capacitor connected between the output of the firstoperational amplifier and the second terminal of the first capacitor. 7.The digitally controlled oscillator of claim 2, wherein the summingcircuit comprises a first input, a second input, and, optionally, athird input, wherein the first input is in connection with the firstoutput of the processing unit and the second input is in connection withthe DC-offset voltage supply, such as the digital-to-analog output ofthe processing unit, and, optionally, the third input is in connectionwith the second output of the processing unit.
 8. The digitallycontrolled oscillator of claim 2, wherein the summing circuit isarranged to form a single pulse wave at a common coupling point based onat least the first pulse wave, a voltage of the DC-offset voltagesupply, and, optionally, the second pulse wave, and wherein theresultant signal is based on the single pulse wave.
 9. The digitallycontrolled oscillator of claim 8, wherein the summing circuit comprisesa first semiconductor switch, such as a MOSFET, wherein the firstsemiconductor switch is arranged to be controlled by its controlterminal by the first pulse wave, and wherein a first terminal of thefirst semiconductor switch is connected to a second voltage supply and asecond terminal of the first semiconductor switch is connected to thecommon coupling point.
 10. The digitally controlled oscillator of claim8, wherein the summing circuit comprises a second semiconductor switch,such as a MOSFET, wherein the second semiconductor switch is arranged tobe controlled by its control terminal by the second pulse wave, andwherein a first terminal of the second semiconductor switch is connectedto a third voltage supply and a second terminal of the secondsemiconductor switch is connected to the common coupling point.
 11. Thedigitally controlled oscillator of claim 1, wherein the summing circuitcomprises a digital buffer arranged between the common coupling pointand the linear wave shaper; or an inverting amplifier arranged betweenthe common coupling point and the linear wave shaper.
 12. A synthesizermodule comprising at least one digitally controlled oscillator accordingto claim 1, and a user interface for controlling the at least onedigitally controlled oscillator, wherein the user interface is at leastin functional connection with an input of the at least one digitallycontrolled oscillator.
 13. A synthesizer comprising at least onedigitally controlled oscillator according to claim 1, a keyboard, suchas a musical keyboard, including a plurality of keys, wherein thekeyboard is at least in functional connection with an input of the atleast at least one digitally controlled oscillator, and, optionally, aspeaker in connection with the output of at least at least one digitallycontrolled oscillator.
 14. A method for producing an electrical audiosignal, wherein the method comprises: generating a first pulse wave by adigital processing unit, the first pulse wave comprising pulses at atleast two different first frequencies; producing a resultant signalbased on at least the first pulse wave; and filtering the resultantsignal by a linear wave shaper comprising an integrator for generatingthe electrical audio signal including, as fundamental frequencycomponents thereof, frequency components corresponding to the at leasttwo different first frequencies, wherein the filtering comprises atleast integration of the resultant signal.
 15. The method according toclaim 14, comprising generating a second pulse wave by the digitalprocessing unit, the second pulse wave comprising pulses at at least twodifferent second frequencies; and producing the resultant signal basedon at least the first pulse wave and the second pulse wave.